Soaring Migratory Birds Avoid Wind Farm in the Isthmusof Tehuantepec, Southern MexicoRafael Villegas-Patraca1, Sergio A. Cabrera-Cruz1*, Leonel Herrera-Alsina2

1 Red de Ambiente y Sustentabilidad, Instituto de Ecologıa A.C., Xalapa, Veracruz, Mexico, 2 Centro de Investigaciones en Ecosistemas, Universidad Nacional Autonoma de

Mexico, Morelia, Michoacan, Mexico

Abstract

The number of wind farms operating in the Isthmus of Tehuantepec, southern Mexico, has rapidly increased in recent years;yet, this region serves as a major migration route for various soaring birds, including Turkey Vultures (Cathartes aura) andSwainson’s Hawks (Buteo swainsoni). We analyzed the flight trajectories of soaring migrant birds passing the La Venta II windfarm during the two migratory seasons of 2011, to determine whether an avoidance pattern existed or not. We recordedthree polar coordinates for the flight path of migrating soaring birds that were detected using marine radar, plotted theflight trajectories and estimated the number of trajectories that intersected the polygon defined by the wind turbines of LaVenta II. Finally, we estimated the actual number of intersections per kilometer and compared this value with the nulldistributions obtained by running 10,000 simulations of our datasets. The observed number of intersections per kilometerfell within or beyond the lower end of the null distributions in the five models proposed for the fall season and in three ofthe four models proposed for the spring season. Flight trajectories had a non-random distribution around La Venta II,suggesting a strong avoidance pattern during fall and a possible avoidance pattern during spring. We suggest that a nearbyridgeline plays an important role in this pattern, an issue that may be incorporated into strategies to minimize the potentialnegative impacts of future wind farms on soaring birds. Studies evaluating these issues in the Isthmus of Tehuantepec havenot been previously published; hence this work contributes important baseline information about the movement patternsof soaring birds and its relationship to wind farms in the region.

Citation: Villegas-Patraca R, Cabrera-Cruz SA, Herrera-Alsina L (2014) Soaring Migratory Birds Avoid Wind Farm in the Isthmus of Tehuantepec, SouthernMexico. PLoS ONE 9(3): e92462. doi:10.1371/journal.pone.0092462

Editor: Brock Fenton, University of Western Ontario, Canada

Received November 29, 2013; Accepted February 21, 2014; Published March 19, 2014

Copyright: � 2014 Villegas-Patraca et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permitsunrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: This research was made possible with funding from the World Bank to the Comision Federal de Electricidad (CFE) for monitoring the environmentalimpacts of the wind farm. The funders of this study were the CFE. The funders had no role in study design, data collection and analysis, decision to publish, orpreparation of the manuscript.

Competing Interests: The authors have declared that no competing interests exist.

* E-mail: sergio.cabrera@inecol.mx

Introduction

The production of wind energy is increasing rapidly worldwide:

in mid- 2013, there was a total of 296 GW of installed capacity,

but it was expected to grow for a total of 318 GW for the full year

[1]. The Isthmus of Tehuantepec, southern Mexico, is the region

with the greatest potential for wind energy yield in the country [2].

It has been estimated that around 2000 MW of wind power could

be harnessed in the La Ventosa region alone [3]. Consequently,

the 83.3-MW La Venta II wind farm was installed in 2007. By the

end of 2012, a total of 15 wind farms were operating in the region,

producing 1331.65 MW of energy [4]. However, these installa-

tions are located along an important migratory route for raptors

that traverses Mexico [5], leading to concerns about the potential

impacts of wind farms on birds, as some species of diurnal

migrants are commonly observed (particularly during their fall

migration), soaring above a ridgeline which in close proximity to

La Venta II [6], [7], probably making use of the wind updrafts

generated by the walls of the ridge [8].

Birds demonstrate a range of responses to wind farms. For

instance, Martınez-Abraın et al. [9] recently suggested that

vultures may exhibit a form of behavioural learning to avoid

turbines. In comparison, Devereux et al. [10] showed that the

positioning of turbines in two wind farms located in East Anglia

(England) had no effect on the distribution of some species of wild

birds (mostly passerines) occupying agricultural areas. Such

observations have led authors to suggest that wind turbines do

not represent a serious problem to birds [11], [12]. However,

threats that wind farms pose to birds have been catalogued into

four main categories: (1) risk of collision, (2) displacement due to

disturbance, (3) habitat loss and (4) barrier effect [13], [14], [15].

One form of displacement is when birds adjust their migratory

routes (also termed flyways) or local flight paths to avoid wind

farms [13]. For instance, it was reported a significant decrease in

the number of common eider flocks entering the Nysted offshore

wind farm area (Denmark) after the onset of operation [16].

Furthermore, it has been observed that common eiders avoid

flying close to or in the area of the Tunø Knob wind park in

Denmark [17]. Similarly, Garvin et al. [15] documented a decline

in abundance of resident raptor species during the post-construc-

tion stage of a wind farm in Wisconsin (USA). de Lucas et al. [18]

showed that soaring birds (e.g. Griffon Vulture Gyps fulvus, Black

Kite Milvus migrans and White Stork Ciconia ciconia) detect and avoid

the presence of wind turbines of a wind farm in Tarifa (Spain)

better when these were functioning. However, more research is

needed because bird species exhibit a range of responses to wind-

energy facilities, with other factors, such as site and season, also

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playing a role. Specific studies are necessary to assess the response

of birds to different types of wind farms in different locations.

Hence, in the current study, we documented the flight

trajectories of soaring birds in the vicinity of the La Venta II

wind farm in southern Mexico, during both the spring (northward

passage) and fall (southward passage) migratory seasons of 2011.

This work provides preliminary insights about the potential

relationship between the flight trajectories of migrating soaring

birds and the topography surrounding a wind farm.

Materials and Methods

Study AreaThe La Venta II wind farm is located less than 1 km north of

Ejido La Venta, which is a small town on the Pacific slope of the

Isthmus of Tehuantepec, a narrow region that separates the Gulf

of Mexico from the Pacific Ocean (Fig. 1). The Isthmus is an

important corridor for migratory birds moving between North and

South America [19] and an important stopover site for migratory

birds in the fall [20]. During construction of the wind farm, the

area was described as a world-class bird migration corridor [21].

The La Venta II wind farm has 98 turbines arranged in four rows

that are aligned from west to east, with a total nominal capacity of

83.3 MW distributed in a 9.49-km2 area. The farm is located on

the inland edge of the Pacific coastal plain. The wind facility is

quite close to an orographic (mountain chain) system. Specifically,

it is located less than 2 km from the southeast tip of the Sierra de

Tolistoque range, which is a small ridgeline with a maximum

altitude of 700 m above sea level (ASL) running in a west-east

direction. In addition, the wind facility is located ,3 km southwest

of some low-altitude hills and light slopes [7].

Radar EquipmentWe used an X band marine radar (Model FR-1525 Mark 3,

Furuno, Nishinomiya, Japan) mounted on a truck adapted to serve

as a mobile unit. Similar radar laboratories have been described by

Cooper et al. [22] and Harmata et al. [23]. The radar transmitted

at a frequency of 9,140 MHz through a 2-m-long slotted

waveguide (antenna), with a maximum output of 25 kW and

was operated with a pulse length of 0.07 ms. The display unit had a

range resolution of 35 m. The antenna emitted a beam with a

width of 1.23u (horizontal) 620u (vertical), with side lobes 610u[22]. The unit was powered with a low-noise electric generator.

Study Design and Data CollectionWe observed the movements of soaring birds during two

migratory seasons (spring and autumn of 2011) using a single

marine radar within the wind farm, sited on the service road of the

northern-most row of turbines (16.596992 uN latitude, 2

94.811981uW longitude, 14 m ASL) where the surrounding

vegetation served as a radar fence. Observations started at

approximately 09:00 and included 4 to 6 continuous 1-h sampling

sessions per day for 13 days in spring (between March 31st and

April 29th) and for 15 days in fall (between October 5th and 25th).

These dates and times of the day coincided with the known peak of

diurnal migratory activity of raptors in the vicinity of the La Venta

II wind farm (Villegas-Patraca, unpublished data). Each hourly

session was subdivided into: 1) 10 min to adjust the radar, 2)

20 min to observe and collect data on the flight trajectories, 3) a

10-min break and 4) 20 min to continue collecting data.

From the radar display, we recorded the flight directions and

three sets of polar coordinates for every trajectory (i.e., the start-,

end- and mid-points), which were measured with a compass and

index line from the screen. All data were recorded manually onto a

laptop computer. We used the term ‘‘target’’ to designate objects

detected by the radar because it did not allow unequivocal

identification. However, based on concurrent direct observations

from a hawk-watch monitoring station in both seasons (Fig. 1), we

confirmed that most of the detected targets on the radar were

either individuals or flocks of soaring birds. The radar was

operated in surveillance mode, with a 6-km detection radius. This

setting has been proven useful for the detection of soaring birds

when using this type of radar [22] (and personal observations). We

did not measure flight altitudes. Data is available at http://dx.doi.

org/10.6084/m9.figshare.938235.

Our study did not involve handling bird specimens. The only

permit needed was to access the wind farm, which was kindly

provided by CFE (Comision Federal de Electricidad). No further

permits were required for the described monitoring.

Data AnalysisWe loaded the polar coordinates into R 2.15.1 [24], on which

the flight trajectories of each season were plotted by joining their

start-, mid- and end-points. We also plotted the polygon showing

the perimeter of the wind turbines of La Venta II wind farm and

then estimated how many times this polygon was intersected by

the documented flight trajectories. Then, we ran simulations of the

documented flight trajectories 10,000 times under different

scenarios or null models for each season (Table 1). To obtain an

index that was comparable between the real and simulated data,

we divided the number of intersections by the total length of all

trajectories (in km) and, finally, constructed a frequency distribu-

tion of the intersections/km for both the observed and simulated

trajectories. If the observed number of intersections/km fell in the

250 smallest or largest values of the distribution from the simulated

data, the hypothesis of randomness was rejected at a= 0.05. We

analysed the directions of trajectories with the circular statistics

software Oriana ver. 4.01 [25], reporting the mean flight direction

(m) and the length of the main vector (r). We also report the species

of soaring birds observed from the hawk-watch monitoring station,

and their abundances.

Results

FallDuring the fall season, we recorded 193 flight trajectories, with

a total length of 1,447.68 km. The mean flight direction (m) was

143.9u, with the observed trajectories being closely clustered

around the mean (r = 0.91), supporting the expected flight

direction for fall, when Nearctic-Neotropical migrants fly south

to their wintering grounds (Fig. 2). The most abundant soaring

bird species identified by the hawk-watch station was the Turkey

Vulture C. aura (n = 266,977), followed by Swaninson’s Hawk B.

swainsoni (n = 66,545); hence, we assumed that most of the

trajectories were from flocks of these species, though the hawk-

watch station identified several others (Table 2). During the

autumn season, the polygon defined by the rows of wind turbines

was intersected 90 times by the flight trajectories, resulting in a

total of 0.0621 intersections/km. This index was lower than that

obtained under the five simulated scenarios or null models

(Table 3).

SpringDuring the spring season, we recorded 87 trajectories, with a

total length of 257.47 km. The mean flight direction was 184.8u (m)

which differed to the expected flight direction for spring, when

Nearctic-Neotropical migrants fly back to their breeding grounds

in North America; however, the observed trajectories were widely

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scattered, showing a low concentration around the mean (r = 0.42,

Fig. 2). The most abundant soaring bird species identified at the

hawk-watch station was the Franklin’s gull Leucophaeus pipixcan,

followed by the Turkey Vulture C. aura and Black Vulture Coragyps

atratus, (n = 13,402, 1286 and 1174 respectively; Table 2). During

the spring season, the polygon representing the wind turbines was

intersected 28 times by the flight trajectories, resulting in a total of

0.1087 intersections/km. This index was lower than that obtained

under Models 1, 2 and 3, leading us to reject the hypothesis that

the number of observed intersections was random under these

Figure 1. Study area. (a) Isthmus of Tehuantepec in southern Mexico, (b) location of the La Venta II wind farm in Oaxaca, (c) arrangement of windturbines, surrounding topography, and nearby towns. Black square within the wind farm (c) shows the location of the radar monitoring station inboth seasons; black triangles show the location of the hawk-watch stations in fall (,700 m northeast of radar) and spring (,2 km southwest).doi:10.1371/journal.pone.0092462.g001

Table 1. Different models or scenarios used to simulate the flight trajectories.

Model Restrictions

1 None. Start-, mid-, and end-points were randomly generated to obtain the same number of trajectories as we observed in each season, resulting incompletely random trajectories within the detection radius, and in a very broad model

2 We retained the observed start- points, randomly generating the mid- and end-points. This scenario simulated trajectories that followed new paths (norestriction of direction and length), but started from the same start- points as the observed ones.

3 We retained both the observed start- and mid-points, and randomly generated the end-points. This scenario simulated alternative trajectories, after the‘‘simulated flocks’’ had passed through the observed start- and mid-points. We did not restrict the length or the direction of the trajectory between theobserved mid- and simulated end-points

4 We retained the observed start- and end- points; however, the mid-point was randomly generated in the rectangular space formed by the intersection ofimaginary lines extending from the x- and y-axis of the start- and end-points. This scenario simulated alternative trajectories between the observed start-and end-points

5 We only applied this model to the fall season dataset. We retained both the observed start- and mid-points, randomly generating the end-points south ofthe former two. This scenario considered the seasonal tendency of flight directions, simulating alternative endings of the trajectories after the ‘‘simulatedflocks’’ had passed through the observed start- and mid-points. We did not restrict the length of the trajectory between the observed mid- and simulatedend-points. We did not apply this model to the spring data because flight directions did not show a marked pattern.

doi:10.1371/journal.pone.0092462.t001

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scenarios. In comparison, Model 4 did not depart from the null

distribution (Table 3).

Discussion

Different responses by birds to the presence of wind farms have

been observed around the world [9], [10], [15], [26]. Here we

present the first report of potential avoidance behaviour for a

Mexican wind farm. La Venta II was inaugurated in 2007 [27],

becoming the first large operational wind farm in Mexico.

However, there has been a rapid increase in the number of wind

energy developments in the region, which is part of one of the

most important bird migration routes in North America [5] and is

known to support large numbers of Swainson’s Hawks B. swainsoni

Figure 2. Flight trajectories and directions. Each line in left panels represent the flight trajectory of one flock, black triangles at the centre showsthe location of the radar monitoring station. Left panels represent a summary of flight directions. Upper panels (a) autumn, lower panels (b) spring.doi:10.1371/journal.pone.0092462.g002

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and Turkey Vultures C. aura during the fall migratory season [28],

[29], whereas the most common species of soaring bird in spring is

the Franklin’s Gull L. pipixcan.

Avoidance behaviour has been observed in coastal [26], inland

[15], [18] and offshore [16], [30] wind farms. But many factors

influence whether birds avoid or enter wind energy facilities [31],

[32]. Hence, research at local-scales is required to evaluate how

birds respond to site-specific conditions. Our goal was to provide

the first documented accounts of how soaring birds responded to a

wind farm in the Isthmus of Tehuantepec. In spring and fall, the

observed number of intersections/km fell outside of the null

distributions obtained under Models 1, 2 and 3. Even the

completely random simulated trajectories intersected the wind

farm more often than the real observed flight trajectories. This

difference between expected and observed trajectories indicates

that birds were exhibiting an avoidance pattern of movement. The

same conclusion was obtained from Models 4 and 5, which had a

highly restricted design, during the fall season. In these two

models, our results again showed a lower rate of actual

intersections/km compared to that obtained under the simulated

scenarios. Conversely, in spring, Model 4 did not depart from the

null distribution. In this instance, our results indicated that the

observed number of intersections/km might be random, with no

pattern of avoidance. Although our results indicate that migrating

Table 2. Species and abundances of soaring birds identified from the hawk-watch monitoring station.

Spring Fall

Species No. Species No.

Leucophaeus pipixcan 13402 Cathartes aura 266977

Cathartes aura 1286 Buteo swainsoni 66545

Coragyps atratus 1174 Buteo platypterus 8685

Mycteria americana 59 Pelecanus erythrorhynchos 1701

Caracara cheriway 36 Mycteria americana 875

Buteo albicaudatus 26 Falco sparverius 119

Buteo swainsoni 19 Coragyps atratus 95

Buteo magnirostris 11 Accipiter sp. 38

Fregata magnificens 9 Caracara cheriway 20

Falco sparverius 7 Buteo albicaudatus 16

Buteo platypterus 4 Pelecaniforme 14

Buteo nitidus 3 Circus cyaneus 13

Buteo brachyurus 2 Fregata magnificens 12

Circus cyaneus 1 Accipiter striatus 9

Falco columbarius 1 Buteo jamaicensis 6

Falco peregrinus 1 Accipiter cooperii 4

Pandion haliaetus 1 Buteo nitidus 4

Falco femoralis 4

Falco peregrinus 3

Falco sp. 3

Buteo brachyurus 2

Buteo albonotatus 1

Buteo magnirostris 1

Elanus leucurus 1

Pandion haliaetus 1

doi:10.1371/journal.pone.0092462.t002

Table 3. Summary of the null model results.

Intersections/km

Observed Model 1 Model 2 Model 3 Model 4 Model 5

Spring 0,108 0,147–0,201 0,145–0,196 0,142–0,193 0,098–0,142 -----*

Fall 0,062 0,156–0,193 0,140–0,173 0,155–0,141 0,091–0,116 0,132–0,157

Ranges represent 95% of the number of intersections/km from 10,000 trajectories under the simulated scenarios or null models for the spring and fall seasons. Bold typeindicates ranges that do not include the observed number of intersections/km (p,0.05).*Not evaluated.doi:10.1371/journal.pone.0092462.t003

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soaring birds avoided La Venta II, we lack of comparable data

from the pre-construction stage, hence we cannot really assess if

the observed patterns are a response to the presence of the wind

farm. However, a 5-days survey made during one fall season

before La Venta II was built, suggests a similar pattern of flight

trajectories as reported in this study [6].

The pattern observed in fall might be explained by geographical

features. For instance, the Sierra de Tolistoque is a ridgeline

located to the northwest of the wind farm, where the interaction of

the wind with its walls may generate valuable resources (updrafts)

which may provide suitable airspace habitat [33] temporarily used

for continued soaring flights. Flocks of raptors are commonly

observed flying above the ridge during fall [6], [7], and most of the

flight trajectories recorded during fall in this study started from this

location. Given the position and (west-east) orientation of the

Sierra de Tolistoque, it is possible that the observed flight

trajectories of soaring birds in fall did not intersect La Venta II

because the ridgeline naturally guided the birds away from the

farm. In spring, soaring birds are expected to approach the wind

farm from the south, where a close prominent ridge is not

available to lead the direction of birds. In that season we recorded

less flight trajectories, but most of them were clustered on the west

side of La Venta II, close to Sierra de Tolistoque, suggesting again

that this ridgeline may be playing an important role in the

observed pattern, probably as a landmark used by soaring birds to

guide their journeys.

Our results suggest that La Venta II do not represent a serious

threat to migrating soaring birds, hence we consider it to have a

fortunate location, as it was decided considering mainly the

availability of the wind resource [6]. Other studies have obtained

similar conclusions, leading some authors to suggest that wind

farms do not represent a substantial risk to birds [12]. However,

we need to warn that the observed pattern might be site-specific to

La Venta II, with the surrounding geographical features playing a

significant role. Therefore, similar studies are urgently required at

the other wind farms located on the Isthmus, as all are sited on the

same migratory route. This requirement is particularly important,

as previous studies have shown that raptors behave differently at

different sites, even when in close proximity [26], and because the

Mexican government aims to increase the production of clean

energy during the next years, potentially including the installation

of .200 wind farms in the country [34].

Although it has been suggested that collision-related fatalities do

not have an effect of populations of birds [35], a recent study

estimates that a mean of 234,000 birds are killed annually by

collisions with wind turbines in the contiguous United States alone

[36]. Hence, although our results show that La Venta II represents

a low risk to migrant soaring birds, further and continued studies

are necessary considering the potential cumulative impacts that

several wind farms clustered on the Isthmus might have on

migrant birds; besides, La Venta II has an expected useful life of

,20 years [6], which might be similar for other nearby wind

farms. This and the above mentioned plans for future energy

production in Mexico, suggest that wind farms in the Isthmus are

not to be removed from the landscape in the close future, but the

opposite. Furthermore, the effects of La Venta II on resident bird

species should also be evaluated. Such work is important, because

it has been suggested that this particular wind farm may become a

local population sink for resident species such as the White-tailed

Hawk Buteo albicaudatus, as some carcasses of this species have been

found within this wind farm [21], and because we have found

more carcasses of resident than of migratory species (unpublished

data).

Although our study may be technically simple, our results

advance existing knowledge about how soaring birds respond to

wind farms in this particular area, which is highly used by some

species of migrant soaring birds. Furthermore, we suggest that

geographical features play a potentially important role in aiding

soaring birds to avoid wind farms, an issue that may be considered

by decision-makers and wind-energy developers in the region as

part of their strategies to minimize the negative impacts of future

wind farms on birds. However, we also highlight the need for

continued and improved studies in the Isthmus, to raise the much

needed information for the region.

Acknowledgments

We thank the Comision Federal de Electricidad (CFE) for allowing us to

conduct the monitoring work within the La Venta II facilities. Thanks to

Rodolfo A. Lopez-Polanco for his great work collecting data, and to

Gabriela Alva-Alvarez who helped to generate the Figures. Thanks to Falk

Huettmann and an anonymous reviewer for their valuable comments.

Author Contributions

Conceived and designed the experiments: SCC RVP LHA. Performed the

experiments: SCC. Analyzed the data: LHA SCC. Contributed reagents/

materials/analysis tools: RVP. Wrote the paper: SCC LHA RVP.

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PLOS ONE | www.plosone.org 7 March 2014 | Volume 9 | Issue 3 | e92462